Metals and alloys thereof are used in nuclear fuels to increase burn up (fuel utilization) and to gain beneficial chemical, thermal, and mechanical properties over pure or combined actinide fuels, such as uranium (U) and plutonium (Pu). For example, an additive is often mixed with uranium to form a pseudo-binary mixture “U-xM,” where “M” is a metallic additive, and “x” conveys relative amount information. The metallic additive can be an elemental metal, an alloy of metals, or a mixture of metals. Typically, “x” refers to the weight of the additive as a percentage of the whole U-xM mixture. More generically, U-M refers to a mixture having uranium and a metallic additive without indication of relative amounts or whether the metallic additive is a single metal or an alloy or mixture of metals.
Zirconium (Zr) in particular is added to actinide metal fuels to produce mixtures having relatively high solidus temperatures, below which only the solid phase is present. The liquid phase is prohibited in nuclear fuel materials by, for example, regulations of the Nuclear Regulatory Commission (NRC) of the United States. Zirconium additives are also believed to reduce the chemical reactivity of a fuel mixture with materials such as iron (Fe), which is typically present in the steel cladding of a fuel rod. For example, a cylindrical slug of U—Pu-10Zr has been encased in steel cladding to produce a fuel rod.
Despite that the chemical components of a nuclear fuel slug are typically uniformly mixed prior to service in an induced fission environment, thermodynamic processes ultimately govern the spatial arrangement of the components, which may migrate even when only the solid phase is macroscopically present. Zirconium, for example, can migrate along temperature gradients to the hottest zone, if hot enough to produce a phase with high solubility for Zr, typically the central core of a cylinder, within an in-use fuel slug such that a central zirconium-rich region is formed and a corresponding radial zirconium-depleted region appears over time. Whatever the geometry, any redistribution of the chemical components of a nuclear fuel slug inhibits accurate modeling and complicates efforts toward designing and implementing a nuclear fuel rod and predicting its performance over time.